Isoleucine uptake by intact diploid human fibroblasts grown on coverslips and by membrane vesicles prepared from diploid human fibroblasts will be evaluated. Additional cell lines with aberrant uptake will be isolated and those cell lines already isolated further studied. Kinetic analysis of isoleucine uptake by cell lines lacking specific transport systems will allow separation and genetic classification of the multiple transport systems for isoleucine. Chemical and enzymatic studies of the cell lines with altered isoleucine uptake may allow better definition of the isoleucine binding sites on fibroblast membranes and my lead to the identification of a specific binding substance.